As the rate of information exchange increases, the portion of the electromagnetic spectrum used for wireless communications has become increasingly more crowded. The large number of individuals and entities that wish to use the spectrum to transmit and receive data has led to the institution of assigned channels, or frequency bands, dedicated to carry particular kinds of traffic and designed to minimize interference among the different transmissions. The growing number of frequency bands requires the bands to be spaced together as closely as possible in order to maximize the spectrum's capacity to carry information. This dense spacing means that the noise floor of transmitters, receivers, and other communication system components must be kept low enough that the noise generated by the components does not overwhelm other components of nearby systems. A component with a high noise floor will tend to desensitize any nearby receiver operating in that component's frequency band.
The collocation requirements of various frequency bands can be quite strict. In general, fixed-site base station radios require a very low noise floor to meet the collocation requirements of the various transmitter-receiver installations. The Air Traffic Control (ATC) band, located approximately between 118 MegaHertz (MHz) and 137 MHz, is just one example of a frequency band that demands low noise. A voltage-controlled oscillator (VCO), part of the frequency synthesizer in radio communications equipment, is found in both transmitters and receivers. The VCO generally sets the noise floor of the radio, whether it be the transmitter noise floor or the receiver local oscillator noise floor, both of which have an effect on collocation. Current technology attempts to achieve the required low noise floors by making use of bulky, expensive, manually-tuned filters to filter the frequency source. Accordingly, a need exists for a VCO capable of meeting the collocation requirements of various frequency bands without requiring manually tuned filters.